Airblast fuel nozzles for gas turbine engines typically have an injector with generally concentric chambers for inner and outer air flow and intermediate fuel flow, and generally concentric discharge orifices for discharging and intermixing the inner and outer air flows and fuel flow in the combustor. A tubular extension or support strut extends from the head of the injector for attachment to the casing of the engine to support the tip of the injector relative to the combustor casing. A central fuel passage extends from a fuel pump through the extension to supply pressurized fuel to the injector. Helmrich, U.S. Pat. No. 3,684,186; Simmons, et al., U.S. Pat. No. 3,980,233; Halvorsen, U.S. Pat. No. 4,902,889; Halvorsen, U.S. Pat. No. 4,754,922; Halvorsen, U.S. Pat. No. 5,014,918; and Mobsby, U.S. Pat. No. 4,170,108 describe and illustrate this type of airblast fuel nozzle.
Airblast fuel nozzles have employed a valve upstream in the fuel passage leading to the injector head (and outside the combustor case) to compensate for pressure head effects and provide adequate fuel distribution to the engine combustor. Although fuel back pressure is thereby maintained up to this valve, this valve can be considerably upstream from the tip (discharge orifice) of the injector. This can cause fuel at low pressures and velocities downstream of the valve to vaporize and/or coke at high fuel temperatures. Fuel vaporization and coking in the injector head can cause pulsing or intermittent interruptions in fuel flow, limit or prevent fuel flow, and in general, cause combustion instability and adversely affect the operation of the engine.
Airblast fuel injectors have been developed in an attempt to reduce fuel vaporization and coking at elevated fuel temperatures. Some injectors have a valve within the injector head which is closed when fuel pressure is below a minimum selected value, and open when fuel pressure exceeds this value. Halvorsen, U.S. Pat. No. 5,014,918, shows such an injector where an arcuate seat is formed in an annular fuel chamber between an inner and outer air chamber in the injector, and an arcuate spring valve is disposed in the valve seat. The arcuate spring valve opens after the cracking pressure of the valve has been exceeded, and closes when the pressure drops below the cracking pressure. The placement of the valve in the injector head maintains fuel back pressure to the nozzle head and can thereby reduce fuel vaporization and coking through at least a portion of the injector.
Other references which show valves in the injector head include U.S. Pat. Nos. 3,598,321; 4,593,720; 5,197,290 (leaf- spring valves); U.S. Pat. Nos. 4,962,889; 5,014,918; 5,174,504; 4,962,889; 4,831,700; 5,754,922 (annular spring valves); U.S. Pat. Nos. 5,102,054; 4,938,417 (tubular metering valves); and U.S. Pat. No. 5,265,415 (internal reed valves).
While the above-described types of injectors increase the fuel flow back pressure through a portion of the injector, and thus can reduce fuel vaporization and coking, they are not without drawbacks. For example, some of the valves in the injector heads are located upstream from the tip (discharge orifice) of the injector, which can still allow vaporization or coking of the fuel to occur between the valve and the tip of the injector.
While injectors have also been developed where a valve is located at the tip of the injector (see, e.g., U.S. Pat. Nos. 2,144,874 and 4,638,636), it is believed that these injectors have been limited to a diaphragm-type of valve which can have a high rate of flow increase (high gain) after the valve cracking pressure is exceeded. A high rate of flow increase through a valve, however, can magnify inconsistencies or variations in stroke effects. It is also believed that the swirl component of the fuel stream in a diaphragm-type of valve is reduced at higher flow rates, which therefore reduces the intermixing of the air and fuel and hence reduces the combustion efficiency of the engine. Thus, a diaphragm-type of valve can be undesirable in some operating conditions.
In any case, it is also believed that the above-described types of injectors can be complicated or difficult to manufacture to precise operating standards, can be difficult (or impossible) to easily tailor or configure to particular engine characteristics, and can have issues with repeatability and dependability over extended use.
As such, it is believed that there is a demand in the industry for an airblast fuel injector for a gas turbine engine which reduces vaporization of the fuel, can be easily tailored or configured to the particular characteristics of the engine to maximize engine efficiency, and is repeatable and dependable over an extended life cycle.